1. Field of the Invention
Embodiments of the invention relate to the field of semiconductor fabrication, and more specifically, to mask blank inspection.
2. Description of Related Art
Extreme Ultraviolet Lithography (EUVL) has become a preferred technique for optical lithography. EUVL typically uses a reflective mask architecture. A defect on the mask may cause defective image of the pattern on the wafer. Therefore, mask blank inspection is necessary to provide qualification of each mask blank before and after shipment to users. Defects in mask blanks need to be analyzed to determine defect criticality.
Existing techniques to address the above problem have a number of drawbacks In actinic inspection, extreme ultraviolet (EUV) radiation is used together with dark field imaging and two-dimensional image sensors. This method may provide sensitive inspection for multilayer defect detection in EUV mask blanks. However, this method relies only on the inspection signal intensity. This is insufficient to decide whether the detected defect is critical or not. Another method uses EUV microscope with large magnification to resolve uncertainty in criticality. However, its effectiveness is not demonstrated. In addition, using an EUV microscope with high magnification is expensive.